1. Field of the Invention
The present invention relates to the field of electrical heating cable typically used to maintain pipes and related equipment at temperatures above freezing or at elevated process temperatures. In particular, the present invention provides an improved parallel zone heating cable with enhanced flexibility and shortened zone lengths.
2. Description of the Prior Art
Parallel zone heating cables are known per se and are in common usage in refineries, chemical plants, commercial and residential installations, and identified as heat tracing cables. In a typical construction of a parallel zone cable, two or three insulated bus wires (also called electrode wires) are provided. They may be solid or stranded, and are typically insulated with polyvinyl chloride, thermoplastic elastomers, fluoropolymers, or any other known and temperature rated conventional insulation. The insulated bus wires are jacketed with a further layer of insulating material, which is provided to maintain the bus wires in a parallel, untwisted configuration, as is necessary for further processing. The insulation over the bus wire insulation is skived in 1 or 2 inch sections, at alternating sites from bus wire to the other, along the full length of the cable, to expose the metal bus wire. A heater wire of known resistance, measured in ohms/lineal foot, is then spirally wrapped around the jacketed bus wires, making electric contact at the alternating exposed sites, with the bus wire. A layer of Fibreglass may then be wound over the heater wire, to secure and cushion the heater wire, and the entire construction is then jacketed with an electrically insulated layer.
The cable described above has been in common use for a number of years and in most conditions will function quite well. However, the heater wire that has traditionally been utilized has been a monofilament wire, and under conditions of rough handling or high temperature cycling tends to break, causing a heater zone (being the distance between two adjacent sites where the insulation has been skived away) to be interrupted, and thereby lose its heating ability. A small number of random zone failures is not considered fatal to a cable, since a zone will be heated by the preceding and following functioning zones, such as on a pipe containing water, oils or chemicals. However, a number of successive zone failures will prevent reasonable operation of the heater cable and will necessitate its removal and replacement.
It has also been observed in parallel zone cables of the sort described above, that thermal shock to the heating wire during the application of an extruded outer jacket may cause the heater wire to form a v-shaped groove along the inner curve of a cable between the bus wires. This is referred to as chevroning and may, in a high temperature thermal cycling environment, result in heat wire kinking and breakage.